Su-47 build/question thread.

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Squirrel member

Hey guys,
I have a project which will need all the help I can get.
I am constructing a DtFB Su-47, and need some advice on stabilizing my wing structure, as well as control surface sizing and bracing.
The first model will be a flat 2d model for proof of concept. 30" wingspan.

Depending on the success of the first, the second should be around a 40" wingspan, full fuse.

Intermediate Hobbyist

Dedicated foam bender

FA, forward swept wings are very cool but particular attention needs to be paid to bracing them, both in torsional rigidity and spanwise. Flutter was a real problem with some of the first ones until the materials caught up strength-wise. How fast are you wanting to fly this? Some strategic use of carbon fiber will save the day, I think.

Dedicated foam bender

Squirrel member

Thanks guys, I read that it's a good Idea to build in washout into the wing tips,and that they have a stress aoa effect that feeds on itself untill the wings snap. I have CF I can use, triangles any one? Edit: Slow! I'm thinking light and manueverable.

creator of virtual planes

When I built a profile X-Wing (yes, from Star Wars) I had a KF step on the top wings and I didn't have any issues with the wings folding. However, my second one I didn't have the KF step all the way to the center and the wings folded a bit at the fuselage because it wasn't braced there at all. The bottom wings on my X-Wing folded right away because I had no KF step or bracing on those. I just used two bamboo skewers to brace it without issues. I flew my X-Wing slow also.

From personal experience I can say that simply adding a KF step to the wings should be enough bracing. Plus you'll have more of an airfoil. So it's a win win.

The CG for my X-Wing ended up being right at the leading edge of the wings. Of course an X-Wing doesn't have forward swept wings, so your CG might be a little forward from the wings. I had a real issue finding the CG. With the wings in back like that, CG is extremely important and very touchy. But starting at the leading edge or a little before should be a good starting point. If it's too far back, your plane will do a back flip. If it's too far forward, you will have no control over it.

EDIT: I should say that CG is extremely touchy because my X-Wing was a pusher, with the motor in the back. I'm not sure if the pickiness of the CG was because of the wings being in the back, the motor being in the back, or both.

Squirrel member

Thanks, a forward CG is definitely the way to go. Except with a wing, then it better be spot on or it always finds the fence post where I launch!:black_eyed: I'm a pretty intermediate scratch builder with about 30-40 aircraft that I have scratch built. However only 4 of those were jets, and of those I had a 75% failure rate. Only one worked.
This is jet is a big leap, but a feasible one, I'll post progress after school.

I think RCSpaceflight had a good plan with the KF step for your wing. I would try to get at least some carbon fiber in there as well due to the possibility of torsional twist, I'm thinking straight across as far forward as you can and then a piece on each wing from near the vertical stabs to the wing tips to form a sort of triangulation.
Ailerons can probably be 3/4-1" running tip to half span or 3/4 span. I would keep the throws low if you go the full inch and 3/4 span. It should be very responsive at those dimensions.

As far as motor location, were you thinking of pusher or prop in slot or something else? I know RCpowers and others have had a lot of success with prop in slot builds...plus it might be easier to balance that way.

I'm also hoping you'll have a lower keel of some sort to brace where the cutout for the canard servo is. That looks like it can be a very weak point.

Squirrel member

Okay! Dan, maybe I could test it as an rc chuck glider before I put a motor on it.
I have a huuuge hill near my house that would be useful for that purpose.

Xuzme, yeah, sorry I haven't finished the belly structure nor the Fuse reinforcement yet.
That bracing is a great Idea. Thanks a lot!
A KF 2 or 3 looks good. Does anyone have experience with a KF 3?
My only KF plane was a trainer with a KF 2 but that was 2 years ago. Have to go dig it out...

I'm not sure how to brace a prop in slot on this plane. I'll go watch the Ft-22 episode.

Squirrel member

More progress! I'm thinking of putting the motor about 3 in. Behind the Canard servo.
Learning from other builds, I installed a 1/2 in balsa nose block under the nose to provide strength and nose weight.
I decided to go with a simple flat bottom airfoil, I know it, I love it, and the last thing I need is another variable.:black_eyed:

Edit: sorry, really bad pic, I can't seem to get rid of it.

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creator of virtual planes

I'm guessing that the CG is going to end up about an inch in front of your airfoil. I agree with CraftyDan that if you do a prop and slot, the closer you have it to CG the better. But if I where you, I'd cut the hole for the prop in the thickest part of the plane so you'll need less bracing. That looks to be close to where the CG will end up being. I don't know if you'll want the motor in front of or behind the prop, that probably depends on how it balances.

Squirrel member

Nice guess! Yes that's where the glide tests have the CG right now.
Thank you, this is one of my best builds so far. How much bracing do you think I will need for the slot when I cut it out? I am thinking about a 7 inch butterfly slot to eliminate noise.

Hostage Taker of Quads

Nice guess! Yes that's where the glide tests have the CG right now.
Thank you, this is one of my best builds so far. How much bracing do you think I will need for the slot when I cut it out? I am thinking about a 7 inch butterfly slot to eliminate noise.

oh, a few peices of 1/2" square steel tube should do nice . . . Oh! you want it to fly too?!?

How wide is the fuse there? planning on pusher or tractor mounting the motor in the slot? I'f you've got some CF arrow shafts, or similar, might run it along the outer bracing 3-4 in forard and aft of the slot (that would be my TLAR, but might be able to get away with less).

Squirrel member

Fuse width is 6" total usable width is 9"' pusher configuration, 6 inch prop, yes I was thinking of running that last forward straight spar through the motor mount
Sorry, I'm still a complete noob at park jets.
By the way, initially I'm going for canard aileron throttle control, followed by elevator.
The canard can have any where between 10 and 165 degrees of throw, I'm thinking of starting around 20 degrees with lots of expo.

Hostage Taker of Quads

Fuse? 6" total usable width, 9"' pusher, yes I was thinking of running that last forward straight spar through the motor mount
Sorry, I'm still a complete noob at park jets.
By the way, initially I'm going for canard aileron throttle control, followed by elevator.
The canard can have any where between 10 and 165 degrees of throw, I'm thinking of starting around 20 degrees with lots of expo.

In all fairness, I've never finished building a Prop-in-slot before either -- I just know how a few have been designed. A 6" fuse is gonna get you, maybe you can scale out a bit (stretch it to 7-8")? There's no rule saying the motor *has* to be on CG, it's just easier to design that way (especially for ARFs where pople like to pick diffrent motors). If you move it farther back you'll have more structure to play with, but you'll have to move other things forward to keep balance.

BTW, With a 2200kv motor, 9" prop seems *awfully* long (running 2S or 3S? 4S?!?). I'd expect something like a 5-6" electric prop, spinning at those RPMs -- won't be hard for a tiny prop at that speed to draw close to 200W.